The AR-15 platform is one of the most popular and widely used assault rifle platforms in the world; particularly in the armed forces of North Atlantic Treaty Organization (NATO) member countries. Technically, AR-15 is a trademark used by the ArmaLite company and later sold to Colt. Colt currently makes perhaps the most famous AR-15 variants, the M16 rifles and the newer M4 carbines. However, a large number of firearm manufacturers now produce firearms understood to be AR-15 variants (e.g., Rock River Arms, Ruger, Stag Arms, etc.). The widespread adoption of the AR-15 standard has lead to the availability of a large variety of interchangeable parts (e.g., barrels, upper receivers and stocks) and accessories, making the AR-15 platform very adaptable and further increasing its popularity. For the purposes of this application, while noting that there are other assault rifles standards (e.g., the AR-10 platform), the term “standard assault rifle” shall be defined to refer to AR-15 rifles and AR-15 variants.
The most standardized portion of a standard assault rifle is the lower receiver (which is also the part considered, under U.S. law, a firearm in its own right). Significant components of the lower receiver include the magazine well, the trigger well, an attachment point for a pistol grip and a collar for receiving a buffer tube of a stock. Two sets of holes are defined through the sides of the lower receiver, with the holes accepting takedown/pivot pins for connection to an upper receiver. The distance between these sets of holes is the same on all standard assault rifles, allows for the ready interchangeability of other components, and has generally delimited the overall size of the lower receiver. Despite the advantages offered by existing standard assault rifles, further improvements are possible.